1. Field of the Invention
The present invention relates to a repeat control apparatus to be adapted to an information reproducing apparatus for reproducing information recorded on an optical disk or an information recording apparatus for recording information onto an optical disk.
2. Description of the Related Art
A repeat control technique is used in various technical fields for the purpose of improving accuracy of control processes.
The repeat control is preferably used for a control system wherein its input signal has a cyclical wave form. The repeat control depends on the periodicity of the input signal. In the repeat control, deviation at the previous cycle is reflected in a control operation at the present cycle.
FIG. 1 shows a basic circuit of the repeat control. In FIG. 1, the basic circuit 300 has a repeat control block 330 and controlled system 350. An input signal r is input to the basic circuit 300. The input signal r is a periodic function having a cycle L, namely: EQU r(t)=r(t-L). (1)
The difference between the input signal r and a feedback signal y is fed as an error signal e into the repeat control block 330. The output of the repeat control block 330 is fed as an output signal v into the controlled system 350. The output of the controlled system 350 is affected by a disturbance d.
Assuming that the cycle L of the input signal r is divided into N time units, as shown in FIG. 2, and the repeat control block generates the output signal v for each time unit, the output signal v at the "k"th time is calculated as follows: EQU v.sub.k (i)=v.sub.k-1 (i)+e.sub.k (i) (2)
where v.sub.k (i) is the output signal at the "k"th time, v.sub.k-1 (i) is the output signal at the (k-1)th time, e.sub.k (i) is the error signal at the "k"th time, and "i" represents any one of time units 1 through N.
Such a repeat control technique can be preferably adapted to an information reproducing apparatus for reproducing information recorded on an optical disk. In the reproducing apparatus, a control signal is optically read out from the rotating optical disk. This control signal is used for a tracking servo control, a focus servo control, etc. If there is eccentricity of the center hole in the optical disk or warp in the surface of the optical disk, an error occurs in the control signal. The cycle of the occurrence of this error corresponds to the cycle of the rotation of the optical disk. Therefore, the repeat control is suitable for compensating such an error. Concretely, the control signal is input to the repeat control system, and the output signal output from the repeat control system is used for the tracking servo control or the focus servo control.
However, if the repeat control is adapted to the reproducing apparatus, the following problem arises.
If there is a defect or obstruction, such as various defect of the optical disk (e.g. a damage of the information pit), cracks, dusts existing on the optical disk, etc., the control signal is temporarily damaged by the defect or obstruction.
The control signal is read out from the optical disk by emitting a light beam to the optical disk and detecting the light beam reflected by the optical disk. Therefore, if there is a defect of the optical disk or an obstruction on the optical disk, the light beam reflected by the optical disk temporarily becomes weak or diffused due to the defect or obstruction. As a result, the control signal is temporarily damaged by the defect or obstruction. Actually, the wave form of the control signal is temporarily distorted due to the defect or obstruction. Such a distorted part of the wave form of the control signal occurs cyclically. However, the distorted part appears only while the light beam is affected by the defect or obstruction. The period that the distorted part appears is relatively short. After the light spot of the light beam has passed at the part that the defect or obstruction exists, the distorted part of the control signal disappears.
If the repeat control performs on the control signal having such a distorted part, the repeat control system recognizes the distorted part of the control signal and memorizes it while the distorted part is appearing. As a result, the influence of the distorted part remains in the output signal output from the repeat control system, after the distorted part of the control signal disappears. Therefore, if such an output signal in which the influence of the distorted part remains is used for the tracking servo control or focus servo control, the accuracy of the tracking servo control or focus servo control become worse.
FIG. 3 shows the control signal to be input to the repeat control system and the output signal output from the repeat control system. In the control signal shown in FIG. 3, the distorted part D1 appears during the period TP. As a result, the output signal is influenced by the distorted part D1, so that the distorted part D2 appears in the output signal. The distorted part D1 of the control signal disappears after the period TP passed. However, the distorted part D2 of the output signal remains after the period TP passed.